The present invention relates to circuitry for sensing physical parameters and, more particularly, to a novel circuit for converting the change in resistance of a resistance bridge, sensing the physical parameter, to the frequency of a waveform, for subsequent processing.
Many applications are known in which a physical parameter, such as temperature, pressure, humidity, air flow, and the like, or the difference in a physical parameter, must be sensed. The magnitude of the parameter is often sensed by a sensor having a changing electrical resistance proportional to the changing magnitude of the sensed parameter. Quite frequently, the change in sensor resistance is small and, if the sensor operates in an electrically noisy environment, the electrical noise tends to mask the changes in sensor output, thereby limiting resolution. Also, it is frequently required to interface the sensor output to process-control apparatus utilizing digital electronics, whereby digital sensor data output is required. Further, it is frequently necessary to transmit the sensed signal over a relatively long path to the apparatus requiring the sensor output information. It is known to overcome these difficulties by converting the sensor output to a waveform having a frequency proportional to the magnitude of the sensor output and to transmit the varying-frequency waveform to the receiving apparatus. Circuitry capable of carrying out the sensed parameter-to-frequency conversion, in cost-effective and highly-accurate manner, is very desirable.